Radio-frequency (RF) powered implantable stimulators and battery powered implantable stimulators are described in the art. See, for instance, U.S. Pat. No. 5,193,539 (“Implantable Microstimulator”); U.S. Pat. No. 5,193,540 (“Structure and Method of Manufacture of an Implantable Microstimulator”); U.S. Pat. No. 5,312,439 (“Implantable Device Having an Electrolytic Storage Electrode”); U.S. Pat. No. 6,185,452 (“Battery-Powered Patient Implantable Device”); U.S. Pat. Nos. 6,164,284 and 6,208,894 (both entitled “System of Implantable Device for Monitoring and/or Affecting Body Parameters”). Each of these patents is incorporated herein by reference in its respective entirety.
A wide variety of medical conditions and disorders have been successfully treated using implantable stimulators. For example, implantable stimulators have been used to treat hearing disorders, urinary urge incontinence, headaches, and various other muscular and neural disorders.
To illustrate, the sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from sensorineural hearing loss are unable to derive any benefit from conventional hearing aid systems.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prosthesis—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function. To facilitate direct stimulation of the auditory nerve fibers, an array of electrodes may be implanted in the cochlea. The electrodes form a number of stimulation channels through which electrical stimulation pulses may be applied directly to auditory nerves within the cochlea.
Hence, an audio signal may be presented to a patient by processing and translating the audio signal into a number of electrical stimulation pulses. The electrical stimulation pulses may then be applied directly to auditory nerves within the cochlea via one or more of the electrodes.
The electrical stimulation pulses generated by implantable stimulators are often biphasic. A biphasic electrical stimulation pulse includes two parts—a negative first phase and a positive second phase. It is often desirous for a biphasic stimulation pulse to be charge-balanced. In other words, it is desirous for the stimulation pulse to include an equal amount of negative charge and positive charge. However, because of inherent imperfections in current sources that generate the negative and positive phases of a stimulation pulse, charge imbalances often occur wherein an unequal amount of positive and negative charge is applied via one or more electrodes.
Over time, charge imbalances may result in excess charge accumulating on the electrodes that are coupled to an implantable stimulator. Eventually, the built-up charge may inhibit stimulation and/or cause device malfunction.
Various approaches have been taken to eliminate charge build-up or accumulation on the electrodes of implantable stimulators. In some systems, DC-blocking capacitors are placed in series with the electrodes in order to block or eliminate long term DC current. However, DC-blocking capacitors require significant printed circuit board (PCB) space. Moreover, the charge build-up will still occur, albeit on the DC-blocking capacitors rather than on the electrodes. As a result, a DC bias voltage across the capacitors may form, which, if sufficiently large, may inhibit stimulation.
Bleed resistors have also been used to eliminate charge build-up. Bleed resistors remove built-up charge on the electrodes by dissipating the charge as heat. However, bleed resistors are relatively large and therefore undesirable in many implantable stimulators. Additionally, bleed resistors effectively reduce the accuracy of the current sources that generate the stimulation pulses because they shunt some of the stimulation current away from the electrodes.